Is Triton Pluto's Twin?

During New Horizons' approach to Pluto, planetary scientists (and Plutophiles) have been trying to predict what the surface might look like. To do this they look at other bodies in the solar system that have the same general properties as Pluto. The surface of a solid planet records the history of its interior evolution, and also reflects the planet's composition. Icy worlds in our solar system seem to have surface features that are a little bit different from the features we see on terrestrial planetslike Earth. So scientists look for an object with a similar size and composition as Pluto.

A good candidate for comparison to Pluto is Neptune's strange moon Triton. The interiorsof both Pluto and Triton are likely composed primarily of water ice over a rocky core, and in roughly similar amounts. Both are a little bit smaller than our own moon (Triton's diameter is 2,700 kilometers, Pluto's is about 2,230 km). Their outer layers also have some amount of non-water ices such as methane, carbon dioxide and nitrogen, which are present on their surfacesand their thin atmospheres of nitrogen, methane and other minor gases replenished from sublimationof ices from their surfaces.

The portion of Triton viewed by Voyager 2 in 1989 at high resolution is geologically unique in several ways. This photomosaic shows Triton's southern hemisphere - the image is centered near 20 degrees north latitude, and Triton's equator runs approximately through the bright swath across the middle of the image.

Images of Triton returned from the Voyager spacecraft show several types of terrain, including volcanic plains, zones of diapiric convection cells (the famous "cantaloupe terrain") and eroded plains. The southern hemisphere is covered in a terrain that defies explanation. A key Voyager observation was that there are very few impact craters, indicating an extremely young surface age of only a few million years!

Terrains observed on Triton. Top: Cantaloupe terrain formed by solid-state convection of the icy crust; center of the volcanic region showing a major edifice and volcanic pit chains; walled plain formed by volcanism or erosional scarp retreat (Sipapu Planitia) Bottom: Smooth plains including a large dark spot formed by erosion; macular terrain possibly formed by scarp retreat of a sublimating volatile layer, with wind-blown streaks of dark material; irregular ridged terrain near the south pole of uncertain origin. (Each scene is 350 kilometers across.)

Another discovery on Triton was the thin geysers erupting from the southern terrains, one of which is shown here erupting from the surface. Images were taken with a resolution of about 1 kilometer. The vent is at right, which forms a broad fan blown westward at approximately 8 kilometers altitude by high winds.

A Stray KBO

Global map of Triton at 650 meters per pixel resolution in three colors (orange, green and blue).

A new global map of Triton using the best Voyager images available gives a preview of features we could see on Pluto. We should not expect Pluto to be a copy of Triton, however. Pluto is much darker than Triton in some areas and there are many other differences. The only way to ascertain Pluto's geologic history, and that of its large moon Charon, is to go there and look up close – one of the key reasons New Horizons was sent to the edge of the solar system.

Triton is in an unusual orbit around Neptune. It orbits in the opposite direction from Neptune's spin – a retrogradeorbit – and its orbit is inclined relative to Neptune's equator. If Triton had formed around Neptune, its orbit should have been in the plane of Neptune's equator, and it would rotate around Neptune in the same direction as the planet.Scientists have suggested that Triton may be a Kuiper Belt Object that wandered too close to Neptune not long after formation and was captured into orbit.

Something had to slow the proto-Triton down as it wandered past Neptune; one theory is that proto-Triton impacted one of Neptune's existing satellites and leftover debris formed Triton. Another possibility is that "gas drag" caused proto-Triton to slow down as it grazed through Neptune's atmosphere. In either case, a tremendous amount of heat would have been deposited in Triton's interior. That heat may not have been enough to explain the active surface we see today, but tidal forces from its lopsided orbit are powerful enough to keep the interior active, leading to eruptions of icy material onto the moon's surface and a redistribution of mass in its interior.

Pluto is a member of the Kuiper Belt, and the giant impact that likely formed Charon may have disrupted Pluto's interior, just as the interior of proto-Triton would have been disrupted when it impacted one of Neptune's satellites. As Pluto cooled down and the mass in its interior redistributed after the impact, interesting geology may have reformed its surface. Because Pluto's composition is likely similar to Triton's, some of the features we see on Triton (such as the "cantaloupe terrain" and wind streaks shown in the figures above) may also exist on Pluto. The dark wind streaks detected by Voyager 2 on Triton's surface were tens to hundreds of kilometers long and preferentially pointed northeast, away from the south pole.